After Rhudaur was claimed by Angmar & armies of Goblins, Hillmen, Black Númenóreans & other units, The Witch-King builds more units to help Angmar destroy a Great Fortress of Arnor of a place knowed as Amon Sûl. Further invasions from Angmar are thwarted by King Arveleg I, who uses the Palantír of Amon Sul to foresee attacks. To proceed in Arnor's destruction, the Witch-king must destroy Amon Sûl and claim the Palantír.
Rogash, Hwaldar, Morgomir, Gorkil, Kaltdrache and the Witch-king are all involved in this battle. The forces of darkness start off by destroying five Mallorn trees that power various defenses of Amon Sûl. After they are destroyed, they proceed in destroying the walls. Reinforcements of Arnor later arrive, but the enemy units wipes them out completely. Then more Reinforcements of Angmar march towards Amon Sûl. Then Angmar forces turn back towards Amon Sûl and begin destroying the structures in the fortress despite heavy resistance. Angmar eventually summons two Mountain Giants and proceeds to attack the watchtower & the Great Fortress, destroying it once and for all.
Though Angmar has destroyed Amon Sûl, Arveleg has escaped with the Palantír. The Witch-King sends Morgomir to retrieve it.